This invention relates to methods of improving the corrosion resistance of zirconium alloys used in light water nuclear reactor environments and, more particularly, to methods of implanting Zircaloy with hafnium ions to improve corrosion resistance and reduce hydrogen absorption in a light water nuclear reactor environment.
Zirconium alloys have been extensively used in nuclear reactor systems due to their optimal low neutron cross-section (low tendency to capture neutrons), generally good corrosion resistance in water and steam, and adequate strength for cladding uranium fuel used in light water reactors.
Furture reactor designs and utilizations are placing more emphasis on ensuring adequate corrosion resistance. Improvements in the corrosion resistance of the zirconium alloy Zircaloy-4, for example, while retaining the benefits of its other favorable properties is a highly desireable goal for advanced applications.
Zircaloy-4 is an alloy composed principally of zirconium (Zr) but also containing, on a weight percent, 1.20 to 1.70% tin, 0.18 to 0.24% iron and 0.07 to 0.13% chromium.
The corrosion of Zircaloy-4 in water or steam occurs by the growth of a zirconium oxide layer produced by the reaction Zr+2H.sub.2 O.fwdarw.ZrO.sub.2 +2H.sub.2. The growth of this layer occurs initially by either cubic or parabolic oxidation kinetics, but after a certain weight gain, the kinetics of growth are linear. The oxide film in general is quite adherent well out into the linear kinetics region. However, at a certain oxide thickness, the oxide adherency is seriously impaired and spalling or "breakaway" occurs. In nearly all present applications, the breakaway region is not reached.
During the corrosion reaction, however, hydrogen is produced. Part of this hydrogen is absorbed by the Zircaloy-4 metal while the balance is carried off in the water or steam coolant. Hydrogen absorbed by the Zircaloy-4 will eventually reach sufficient concentrations so that zirconium hydrides will form in the Zircaloy-4. The hydride formations can lead to degraded ductility and fracture toughness in the Zircaloy-4 at lower service temperature.
Improvements in the corrosion resistance can therefore occur by lowering the rate of corrosion, by eliminating the change to linear kinetics, by extending the time to film breakaway and by reducing the pick-up fraction for hydrogen in Zircaloy-4. Alloy modifications have been looked at from time to time but the generally good corrosion behavior and adequate strength of Zircaloy-2, Zircaloy-4, Zr-1Nb, Zr-2.5Nb and Ozhennite (Zr-Sn-Nb-Fe-Ni) have tended to limit any further development of commercial reactor grade alloys. However, current concerns over nodular corrosion in boiling water reactors (BWR) and over the variable corrosion resistance at extended burnup in both BWR and pressurized water reactors (PWR) generated a need to look at ways to improve existing alloy grades through modified thermomechanical processing and possibly alloy changes. Alloy modifications and thermomechanical treatments, attempted over the years to improve the corrosion resistance of Zircaloy-4 and other zirconium alloys, in general, alter either the chemistry or the structure of the entire material section, or both. This can lead to variability in the effectiveness throughout the material and alter the other properties of the alloy in an undesirable manner.
Ion implantation is generally known as an effective technique for preparing surface alloys of controlled composition without affecting the underlying bulk structure and properties of the metal. Ion implantation is the process by which sufficiently energetic ions are propelled into the surface of a host material (metal, ceramic or plastic), sometimes referred to as a matrix, such that penetration beyond surface layers occurs. This is achieved by accelerating an ion species to energies between 3 to 500 kev with resulting target penetrations to depths of 100 to 10,000 angstroms below the surface. The ion species is accelerated in vacuum through an appropriate electric potential and electromagnetically aligned and collimated to strike the material target. At low energies the ions essentially plate onto the material target and can subsequently be distributed into the target material through an inert ion (such as xenon) bombardment. At high energies, the ion species are driven into the target material creating a distribution or mixing of the implant ion with the target material.
Use of ion implantation for surface modification of surface sensitive properties of materials by changing the mechanical and chemical behavior of the surface layers is now the subject of growing research and application. Numerous surface properties of materials are influenced by surface composition. Such properties friction, wear, hardness, fatigue, corrosion, resistance, electrochemistry, catalysis, decorative finish, bonding, lubrication, adhesion and reflectance. Hence, ion implantation holds promise as a technique for altering such surface properties.
Ion implantation thus offers a method for bringing about novel surface alloy changes without altering the acceptable bulk mechanical properties of the existing zirconium alloys. Because of the potential for non-equilibrium phase structure, alloy conditions can be created which have never been evaluated before due to the nearer chemical equilibrium imposed by traditional high temperature metal processing of zirconium alloys.